Thousands of people are demonstrating in Hong Kong in defiance of a protest ban for the 70th anniversary of Communist rule in China.
Fifteen people were hospitalised during the clashes, as tear gas was fired and petrol bombs were thrown.
Police have also fired live rounds, and there are unconfirmed reports that one protester was shot in the chest.
Nearly four months of protests in Hong Kong have challenged Chinese President Xi Jinping's vision of national unity.
Earlier, the Chinese flag was raised at a special ceremony in the territory.
Security was tight and the 12,000 invited guests watched the event on a live video feed from inside a conference centre.
On what is being described by protesters as a "day of grief", people took to the streets in central Hong Kong and at least six other districts, blocking roads in some areas.
Police fired tear gas and rubber bullets to disperse the protesters, themselves armed with umbrellas, projectiles and petrol bombs. Police also advanced with water cannon.
Protesters are in retreat but have been setting fire to barricades as they go.
The Hospital Authority said 15 people aged between 18 and 52 were injured, with one of them in a critical condition.
At least 15 metro stations and numerous shopping centres in the city have been closed, and some 6,000 officers have been deployed in the territory.
An annual fireworks display had earlier been cancelled.
What is the background to this?
Hong Kong has been a part of China since 1997 but has its own system of law and government – known as One Country Two Systems.
In recent years, there has been increasing opposition to what has been seen as the growing influence of Beijing on Hong Kong's society and politics.
More on security in Hong Kong:
Hong Kong always sees anti-Beijing protests on 1 October, and this year they were expected to be larger than ever, because of the months of unrest triggered by proposed changes to the extradition law.
The changes would have made it possible for China to extradite people to the mainland from Hong Kong, something opponents felt put Hong Kongers at risk of persecution in unfair Read More – Source